1. Field of the Invention
This invention relates to a packaging material suitable for relatively heavy materials such as a roll film for movie among various photographic photosensitive materials, other photosensitive materials, electric parts and the like.
2. Description of the Prior Art
In general, the packaging materials for photosensitive materials are required to satisfy various properties such as gas barrier, moistureproofness, physical strength such as breaking strength, tear strength, impact puncture strength, Gelbo test strength and wear resistance, heat sealing properties such as heat seal strength, hot tack properties and sealability with other materials, antistatic property, slipping character, low dusting characteristics and flatness, as well as light-shielding ability to shield light completely.
It is difficult to satisfy these properties by a single film material, and heretofore, the packaging materials used were composed of plural layers.
The inventor has investigated for improving packaging materials for photosensitive materials, and for example, disclosed a packaging material of which the physical strength is improved by combining two uniaxially stretched films (U.S. Pat. No. 4,331,725). The inventor also disclosed a laminated film, composed of a foamed sheet having a thickness of 0.3 to 2 mm and an expansion ratio of 5 to 50 times and two uniaxially molecular oriented thermoplastic resin films adhered on the both sides of the foamed sheet by melt adhesion directly or through an extrusion laminating adhesive layer so that respective molecular orientation axes crosses each other at an angle of more than 30 degrees, of which the thickness is reduced to 40 to 85% of the total thickness by pressing (U.S. Pat. No. 4,565,733). The laminated film has a great impact puncture strength and Gelbo test strength and little curling, and it is excellent as a packaging material for heavy products.
However, in these packaging materials composed of plural layers, since each film layer is laminated through an adhesive layer or coextrusion, they are expensive. Moreover, they are stiff due to completely integral joining of each layer, and have a great curling as well as small physical strength. In the case of the laminated films where a coextruded T die film or inflation film is laminated, the laminating cost is expensive, and trimming lose is great.